Reengineering Life is a series from Future Human about the astonishing ways genetic technology is changing humanity and the world around us.
In 2016, the Australian Bureau of Meteorology recorded the hottest sea temperatures on record near the Great Barrier Reef, the world’s largest coral reef system. The heat wave lasted for weeks, triggering the worst bleaching event the reef has ever experienced. Nearly 30% of its corals died as a result.
Rising temperatures due to climate change threaten to bring on more coral bleaching and mass die-offs. Coral reefs are home to an estimated 25% of all marine species at some point in their life cycle, protect coastal areas from storms and erosion, and provide jobs to local communities.
In hopes of saving reefs on the brink of collapse, scientists are racing to understand why some coral survive and others don’t. Of particular interest are the genes behind coral survival. Using the gene-editing technique CRISPR, one group of researchers has pinpointed a gene responsible for heat tolerance in coral from the Great Barrier Reef.
The discovery, published this month in the Proceedings of the National Academy of Sciences, could help guide coral conservation efforts and potentially lead to a 23andMe-type test for corals. A genetic test that could tell scientists which corals are most at risk of bleaching, or alternatively, which reefs may be more resistant to it, would be incredibly useful for conservation, lead author Phillip Cleves, PhD, a marine geneticist at the Carnegie Institution for Science, tells Future Human. Some corals, for instance, can live in extreme environments.
“The main problem in the field has been the fact that we haven’t been able to figure out what genes are playing a role in either coral bleaching or coral survival to heat stress,” Cleves tells Future Human. “We didn’t have genetic tools to allow us to ask what their genes do.”
Cleves and his collaborators have figured out how to use CRISPR in coral. Like a pair of genetic scissors, CRISPR allows scientists to delete or edit any gene in organisms’ cells. The technology is widely used in lab mice and is being tried in humans to address a handful of genetic diseases, as well as cancer. But CRISPR is tougher to use in some organisms than in others.
The challenge with applying CRISPR to coral is that the procedure must be done in newly fertilized eggs. But coral spawns just once a year in the middle of the night during a full moon. When water temperatures are just right, mature corals simultaneously release eggs and sperm into the ocean for fertilization. For the past few years, Cleves and other scientists have been making the trip to Australia’s Great Barrier Reef for this annual spawning to collect newly fertilized eggs for experiments.
“It’s definitely a logistical challenge,” he says.
Back in the lab, Cleves and his co-authors used tiny needles to inject the fertilized coral eggs with CRISPR. They used it to remove a gene known to play a crucial role in heat response in other organisms. Next, they let the fertilized eggs develop, then exposed the resulting coral larvae to water at different temperatures.
The edited larvae survived well in water with a temperature of 27 degrees Celsius (about 81 degrees Fahrenheit) but died quickly when researchers increased the water temperature to 34 degrees (93 degrees Fahrenheit). In contrast, the unedited larvae survived well in the warmer water, suggesting the gene is necessary for heat tolerance. Water temperatures around the Great Barrier Reef typically range from 23 Celsius (75 degrees Fahrenheit) in the winter to 29 degrees Celsius (about 84 degrees Fahrenheit) in the summer. The gene, HSF1, had previously not been known to be involved in heat response in coral.
When water gets too warm, corals expel the beneficial algae that take shelter in their tissues. Algae not only gives coral its vibrant color, but it also makes important nutrients that coral needs to survive. Without them, the coral turns white and becomes more susceptible to disease. Bleaching also hinders their growth, as well as their ability to reproduce.
“This study gives us better insight into how and why corals bleach during periods of heat stress, which is definitely beneficial, particularly as we work to understand how corals can adapt to climate change,” Helen Fox, PhD, conservation science director at the Coral Reef Alliance, a California-based nonprofit, tells Future Human.
In 2018, Cleves and his colleagues were the first to report that editing genes in coral was possible. But in that study, the technique didn’t work well enough: They couldn’t get CRISPR to edit enough cells to actually have an effect on the coral’s physical traits. (This is a common problem with CRISPR.) This latest paper shows that gene editing can be used to delete genes and study the effects of the missing gene. Marine biologists have made similar strides using CRISPR in squid.
“Having the ability to remove genes from coral is going to allow us to catalog what genes do in corals,” Cleves says. “That basic knowledge is going to help us understand both the propensity of corals to survive climate change and also the biological mechanisms involved.”
The study also opens the door for instilling heat-tolerant genes into coral. While Cleves isn’t interested in making genetically engineered coral, some of his collaborators are. Last year, Science magazine reported that coral geneticist Madeleine van Oppen, PhD, is exploring the use of CRISPR to make hybrid corals — those that have traits from different species of coral — that could better tolerate warmer temperatures.
In a commentary accompanying Cleves’ paper, van Oppen cautions that coral strains generated with CRISPR would be classified as GMOs in some countries. “Their release into the environment would, very properly, face rigorous regulatory and public scrutiny,” she writes.
Fox thinks genetically altering coral reefs is a risky conservation approach. “Nature is reliably unpredictable and too much human interference could lead to unintended consequences,” she says.
Plus, bleaching isn’t the only threat that coral reefs face. Most reefs are located in shallow water near shore, making them vulnerable to pollution that originates on land and damage from coastal development and fishing. Though banned in many countries, illegal coral mining is also contributing to the destruction of reefs around the world.
For now, van Oppen and Cleves agree that introducing genetically modified coral into the wild shouldn’t be the focus of conservation efforts, but they’re hopeful that CRISPR could reveal other ways to preserve threatened coral species.
“A more scalable conservation approach is to reduce threats to coral reefs and let evolution take its course so corals can adapt to climate change,” Fox says.